Light-emitting diodes (LEDs) that emit colored light have now become so bright that they are used, for example, for traffic signal systems and in the rear lights of vehicles.
For a microscope illumination system, white light is principally desirable. White-light-emitting LEDs already exist. These are not, however, bright enough for this application. A plurality of light-emitting diodes arranged next to one another and emitting red, green, and blue light can be used to generate white light.
DE 37 34 691 C2 presents a solution making possible a variety of illumination types and an intensity control capability with the aid of this type of red-green-blue arrangement of many small LEDs. A bright-field, dark-field, or oblique illumination can be produced by selective local activation of the individual LEDs. The overall intensity of the illumination can be regulated by the fact that either all or fewer LEDs are in operation, or on the other hand only those LEDs that result in a specific configuration. The individual LEDs are not, however, individually controllable; they can only be switched on and off.
To achieve uniformly homogeneous coverage of the illumination aperture, a frosted disk must be introduced in the immediate vicinity of the LED light source.
A further embodiment according to this existing art provides that white light can be generated with a red, green, and blue planar LED light source and a system of dichroic splitters. This illumination apparatus offers a high light intensity, but is relatively bulky. Because of the long optical paths, the light must be collimated using additional lenses. A further disadvantage is the high alignment accuracy of the dichroic mirrors required to ensure good and constant color accuracy.
The disadvantages of tungsten lamps, halogen lamps, etc., as used hitherto for microscopes, are principally high thermal dissipation, high power consumption and short service life, little robustness, large space requirement, and heavy weight (cf. DE 37 34 691 C2, col. 1, lines 7–12).
DE 19 13 711 A presents a solution for uniform light distribution and intensity regulation of a single conventional light source (Planck radiator) by means of fiber bundles. For that purpose, a fiber bundle having a single entrance and exit surface is placed after the conventional light source with diaphragm. No positional allocation of the fibers with respect to the entrance and exit surface exists.
This offers the advantage that the inhomogeneous intensity distribution of the light source image is homogenized at the end of the fiber, and a continuous intensity regulation is produced at the end of the fiber by way of the diaphragm at the fiber entrance.